"I've heard about it, but I've never like ordered it because it's not, like, on the menu," said Fritz's 18-year-old daughter, Margaux. "And so I'm like, 'What if it's not really there? That would be awkward.'"

Lots of fast food places offer things that aren't on the menu. You just have to know to ask for them.

It's "kind of sneaky," said Alex Grace, of Dallas, a sophomore at Booker T. Washington High School.

McDonald's will sell you a Neapolitan milkshake. Wendy's will hit you with a "grand slam" -- four meat patties stacked on a bun, also called a "meat cube."

"I've known different restaurants that do that," said Alex Grace's mother, Jackie. "You have to know about a certain thing that's on the menu. So yeah, I might ask, 'What do you have that's not on the menu?'"

Chipotle will put together nearly anything you can imagine if they have the ingredients, from quesadillas to nachos.